Metastasis is a complex multi-step process, whereby the malignant cells escape from the primary tumor, invade through the basement membrane of the tissue, survive in circulation under the conditions of anchorage-independence, and colonize the foreign microenvironment at a secondary site (Gupta GP, et. al., Cell, 127:679 (2006); Mehlen P, et. al., Nature Reviews Cancer, 6:449 (2006)).
Metastasis is responsible for significant morbidity associated with cancer and accounts for 90% of cancer-related deaths (Gupta GP, et. al., Cell, 127:679 (2006); Mehlen P, et. al., Nature Reviews Cancer, 6:449 (2006)). Despite the development of new treatment strategies, five-year survival rates for patients presenting with distant site metastases from most cancers remain below 30 percent (Society AC. Cancer Facts & Figures, American Cancer Society (2013)). The likely reason for such high mortality due to metastasis is the lack of effective therapeutic agents and early diagnostic strategies that identify and target specifically, the largely drug-resistant, metastatic cells.
To successfully colonize a secondary site, metastatic cells have to find a niche that will support their survival and growth. Such a niche is comprised of the extracellular matrix of a particular tissue, stromal, immune, and other cellular components, and secreted factors.
In vitro methods, such as the scratch assays, transwell migration assays, and invasion assays only evaluate the ability of cells to migrate on or through a solid substratum, and do not recapitulate the anchorage-independence required for metastatic dissemination through the circulation (Kam Y, at. al., BMC Cancer, 8:198 (2008); Kramer N, et. al., Mutation Research/Reviews in Mutation Research (2012); Liang C-C, et. al., Nature Protocols. 2:329 (2007)). Furthermore, the invasion assays, such as Matrigel invasion, fail to account for the differences in the extracellular matrix composition of the primary and secondary sites (Ioachim E, et. al., European Journal of Cancer, 38:2362 (2002)). These shortcomings largely limit the use of the standard in vitro methods to the studies of either dissemination of tumor cells from the primary site or invasion of the secondary organ, but not both. Mouse models of metastasis are equally inadequate for pre-clinical use because they do not faithfully recapitulate human disease.